In order to secure a front visibility as well as to prevent blinding of a driver on an automobile running in the opposite direction, there are regulations on vertical and horizontal illuminating directions of a headlight.
According to the Japanese domestic regulations, there are specified the position of an optical axis (set inclination .THETA..sub.s) and the illuminance of a high beam on a screen 10 meters ahead. However, in order to reduce the space required for measuring them, it is also approved to measure the position of the optical axis and the illuminance on a screen 3 meters ahead for thereafter converting them to values corresponding to those on the screen 10 meters ahead.
Conventionally, there is disclosed a method in Japanese Published Examined Patent Application No. 63849/1988, in which an illuminating pattern of headlight beams illuminated on a screen is pictured, isolux region having illuminance above a predetermined level is located by picture processing, and a geometric center of gravity of the located isolux region is calculated, thereby measuring the position of the optical axis on the screen.
The headlight is so designed that, at a short distance, there is formed an illuminating pattern which is laterally spread and is not symmetrical relative to the optical axis to improve the visibility of a road surface and a road edge and that, at a long distance, the light beams are narrowed. Therefore, light beams not appearing at a long distance are also illuminated on the screen at a short distance, and there is given rise to a difference between that position of the optical axis on the screen 10 meters ahead which is calculated on the basis of the illuminating pattern on the screen 3 meters ahead, and the actual position of the optical axis.
In addition, the measurement of the optical axis is conventionally carried out on a presumption that a light source of the headlight (i.e., a filament) is positioned in a predetermined position. However, there are cases where the actual position of the light source deviates from a set position due to an initial running-in period of suspension members, variations in the pneumatic pressures of tires, assembling inaccuracies of related parts, or the like. This deviation also gives rise to a difference between that position of the optical axis 10 meters ahead which is calculated on the basis of the position of the optical axis on the screen 3 meters ahead, and the actual position of the optical axis.